Uranium constitutes about 8.times.10.sup.31 5 % of the igneous rocks in the earth's crust, primarily in the form of the ores uraninite (UO.sub.2), pitchblende (U.sub.3 O.sub.8) and carnotite (2K(UO.sub.2)UO.sub.4 3H.sub.2 O). The uranium 235 (U.sup.235) isotope, which constitutes about 0.7% of natural uranium, has chemical properties identical to those of the more abundant uranium 238 (U.sup.238) isotope. These naturally occurring uranium isotopes are the primary useful naturally occurring isotopes. U.sup.235 is used mainly in nuclear applications, while U.sup.238 is used in weapons and ordnance applications. However, before the isotopes can be used for these purposes, they must be separated.
Because of their identical chemical properties, it was thought that U.sup.235 and U.sup.238 could not be separated by chemical methods. Consequently, various nonchemical uranium isotope separation methods have historically been proposed. Methods proposed to separate uranium isotopes, specifically U.sup.235 and U.sup.238, have included thermal difussion employing molten uranium, gaseous diffusion through barriers, the centrifugation of gaseous uranium, and electromagnetic processes wherein U.sup.235 ions were deflected more than U.sup.238 ions in a magnetic field.
Uranium separation and enrichment processes, until recently, have been the province of various agencies of the United States government. One commonly used process involves the differential high temperature diffusion rates of the hexafluorides of the uranium isotopes U.sup.235 F.sub.6 and U.sup.238 F.sub.6, which have different difussion rates. A laser-driven enrichment process has also been used in the production of nuclear grade uranium.
Early workers tried to separate the isotopes of uranium by eluting the uranyl ion, UO.sub.2.sup.+2, on cation-exchange resin with various eluant ions. While some minor separations were achieved, the separation factor was too low to be practicable. Larger separation factors were obtained by passing uranyl ions over resin in the uranous ion form.
When an aqueous solution of uranyl chloride encounters a cation exchange bed containing uranous ion, the cation exchange equilibria tend to favor the concentration of the U.sup.238 isotope as sorbed tetravalent uranous ions on the cation exchange bed and to favor enrichment of the U.sup.235 isotope in the aqueous solution of hexavalent uranyl ions. A variety of procedures has been proposed relating to uranium enrichment using a cascade of a great many ion exchange beds because the maximum separation factor per theoretical stage (that length of bed required to establish the equilibrium isotopic partition) is only about 1.0007. None of these numerous proposals, however, was sufficiently attractive to be commercialized.
While the aforementioned processes ultimately accomplish the desired separation and enrichment of the uranium isotopes needed for nuclear applications, they do so at high processing cost. In addition, since the methods currently used to separate uranium isotopes are primarily enrichment processes and do not also purify the uranium, the uranium-containing material to be processed requires extensive preparation to achieve the purity necessary for nuclear grade uranium 235. Uranium 238 does not have to meet the same purity standards as U.sup.235. Both the diffusion cascade process and the centrifugation process now in use have very high energy requirements since both employ high temperature, vapor phase operations to enrich the U.sup.235 isotopes produced. Moreover, the size of the equipment needed to conduct these processes is extremely large, and a very large facility is required to hold this equipment. Finally, the end product of the aforementioned processes is only an enriched and not an isotopically pure product.
U.S. Pat. No. 3,869,536 to James discloses a chromatographic uranium enrichment process wherein a composition having an isotopic distribution of U.sup.235 and U.sup.238 which is different from the uranium isotope distribution of the feed stock is prepared. The uranous-uranyl chromatographic displacement method described therein employs a cation exchange bed requiring a shorter length of each theoretical stage of enrichment than was previously necessary. A series of interconnected columns through which liquid flows downwardly from one column to another to effect the change in isotopic distribution is described for use with this process. Although the James process produces changes in the uranium isotope distribution of the uranium-containing feed stock, there is no suggestion that the production of nuclear fuel quality pure U.sup.235 is an objective. Moreover, the process requires several reaction vessels.
U.S. Pat. No. 3,971,842 to Eubank discloses a process for the separation and enrichment of uranium isotopes which includes a cascade of ion exchange units designed to produce fuel grade uranium. This process, like that of James, employs a cation exchange bed, but one which may move relative to its chamber. In certain embodiments, the ion exchange bed has an annular shape so that the relative movement between the bed and its chamber is achieved by rotation of the annular bed about the axis. A uranium solution is fed to the bed and subjected to oxidation and reduction along sequential regions in the bed so that solutions of uranium withdrawn from the uranium area of the bed have an isotopic uranium distribution different from the uranium feed. In addition, solutions may enter and leave from the sidewalls of the ion exchange bed. The relative movement between the ion exchange bed and chamber column assists in the enrichment of the uranium ions, which occurs as a result of exchange between the aqueous solution and hexavalent uranium and sorbed tetravalent uranium.
The method and system disclosed in U.S. Pat. No. 3,971,842 represents an effective way to separate uranium isotopes and to enrich the U.sup.235 isotope fraction. However, this method requires both complex, bulky equipment and many involved process steps. In addition, the Eubank method is primarily a uranium enrichment process and does not guarantee the production of isotopically pure U.sup.235 suitable for use as nuclear fuel. The many stages and different processing chemicals required to produce U.sup.235 according to this method, moreover, add substantially to the processing cost.
The prior art discloses a multitude of systems and methods for separating various chemical species, including isotopes. U.S. Pat. Nos. 4,490,225 to Lahoda et al. and 4,584,183 to Chiang et al., for example, are directed to the separation of zirconium isotopes. U.S. Pat. No. 4,584,183 discloses the enrichment of a zirconium isotope component of an aqueous solution by a solvent extraction-exchange process that includes an organic phase, while U.S. Pat. No. 4,490,225 uses a laser to irradiate the vapor of a zirconium-containing compound to separate the isotopes. The processes described in these patents, however, are not suggested to be applicable to the separation of any isotopes other than those of zirconium.
U.S. Pat. Nos. 4,764,276 and 4,808,317 to Berry et al. disclose apparatus useful for separating components in liquid solution. A rotating separator body including discreet chambers of ion exchange resin allows the continuous processing of a liquid feed solution. Although the apparatus described in these patents allows an ion exchange type of separation process to be conducted continuously in a single reaction vessel, there is no suggestion whatever in either patent that the purification or separation of uranium isotopes could be conducted according to the processes described therein.
British Patent No. 786,896 also described a continuous separation process for the chromatographic separation of mixtures with two or more components. The mixture is added to a separating medium, such as an ion exchange resin, and the separating medium is moved relative to the added mixture. An elution agent is passed continuously through the medium, and the separated components are continuously collected. However, there is no teaching that the separation and purification of uranium isotopes can be accomplished according to this process.
The prior art, therefore, has failed to provide a simple, low cost method for efficiently and effectively separating and purifying uranium isotopes, which produces uranium 235 of a quality suitable for nuclear applications.